The present invention is directed to an improved method of compression molding a multi-layered golf ball core.
Generally, golf balls have been classified as solid balls or wound balls. Solid balls are typically comprised of a solid, polymeric core and a cover. These balls are generally easy to manufacture, but are regarded as having non-optimal or limited playing characteristics. Wound balls are comprised of a solid or liquid-filled center surrounded by tensioned elastomeric material and a cover. Wound balls generally have good playing characteristics, but are more difficult to manufacture than solid balls.
The prior art is comprised of various golf balls that have been designed to provide optimal playing characteristics. These characteristics include the initial velocity and spin of the golf ball, which can be optimized for various caliber of players. For instance, certain players prefer to play a ball that is softer feeling and have a high spin rate that allows the player to control or xe2x80x9cworkxe2x80x9d the ball. However, balls of this nature tend to exhibit slight decrease in distance due to the high spin rate. Other players prefer to play a ball that has a low spin rate to maximize distance. These balls, however, tend to be hard feeling and difficult to control around the greens.
Methods for producing golf balls having an ideal combination of the above mentioned desirable characteristics have been many. Manufacturers have molded layers around a solid center by placing a preformed center between two blocks of core material in a spherical compression mold, and closing the mold. This process, however, provides little control over the ultimate placement of the center within the golf ball core. Large variations in the location of the center can result and are extremely detrimental to ultimate golf ball performance. Another method that improves the centering of a solid center involves forming two hemispherical polymer cups with two mold halves that, when placed together, create a hollow cavity in which the solid center rests. The two cups are then heated above the curing temperature of the polymeric material, under compression, to form the golf ball core. However, although centering is improved, at certain desirable temperatures (typically higher) and material compositions (low levels of reinforcing polymer), the cups tend to pull away from the surface of the molds, which can result in a slightly off-center solid center because of displaced shell material.
The prior art also provides for the manufacture of double cover golf balls. This is generally accomplished by injection molding a first cover layer followed by a second cover layer, both around a core. This system, however, requires complex injection molds, usually with retractable pins within the mold to properly position the core. Moreover, this system generally works better with and is, therefore, more ideally suited for use with thermoplastic materials.
Therefore, what is desired is an improved method of molding multi-layer cores that employs a center plate for compression molding that ensures properly centered cores, allows cup or mantle layer formation at higher temperatures, and allows the use of improved formulations by reducing the amount of reinforcing polymer required.
The invention provides a method for making a golf ball, particularly suited for golf balls that have a multi-layer core with a solid or fluid center. The method comprises forming a solid or fluid-filled center, and then molding from elastomeric material, preferably of a thermoset material such as a polybutadiene-based material, core parts, such as hemispherical cups, that when combined form a hollow sphere.
The hemispherical cups each have a hemispherical cavity, formed by a combination of a first mold plate having a hemispherical cavity and a second mold plate with a hemispherical protrusion. The first mold plate cavity has a cavity diameter and, additionally, a circumferential groove surrounding the cavity, the groove having an inner and outer diameter. The inner diameter of the groove is preferably greater than the cavity diameter in the first mold plate. The second mold plate has a channel disposed concentrically around the hemispherical protrusion, the channel having an inner, outer, and center diameter. The channel inner diameter is preferably less than the cavity diameter. Additionally, the channel outer diameter is greater than inner groove diameter on the first mold plate.
The hemispherical cups are formed from elastomeric material by placing a predetermined amount of the material in the cavity of the first mold plate, locating the second mold plate juxtaposed to the first plate, and compressing the mold plates to form a cup, having a substantially hemispherical cavity and a lip extending into the groove on the first mold plate. The center is placed between the cups. The cups are then joined around the center to form a substantially spherical core with a substantially concentric center. Since the lip maintains the cups uniformly thick shape, the center is positioned concentrically within the finished core. Finally, a cover is molded around the core.
In one embodiment, the channel has a center diameter that is substantially the same as the cavity diameter. The channel has a depth that is preferably greater than about 0.01 inches. More preferably, the depth is from about 0.01 to 0.05 inches. Most preferably, the depth is from about 0.02 to 0.03 inches. In another embodiment, the channel inner diameter is between about 55% and 99.5% of the cavity diameter. The channel outer diameter is preferably between about 100.5% and 105% of the groove inner diameter.
The channel has a first and second sidewall, each having angles that are less than 90 degrees from horizontal, as measured from the center of the channel. More preferably, the angle is from about 30 to 60 degrees from horizontal. Most preferably, the angle is from about 40 to 50 degrees from horizontal. The channel, therefore, can be defined, in one embodiment, by a truncated cone cross-section. In another embodiment, the first and second sidewalls are each defined by an arc having a radius.
Additionally, the channel has a top width and a bottom width smaller than the top width. The bottom width is preferably from about 0.1 to 0.15 inches. More preferably, the bottom width is from about 0.12 to 0.13 inches.
In a preferred method according to the invention, the two cups are a thermoset material and the step of joining the cups comprises elevating the temperature of the cups to cause cross-linking. In the step of joining the cups further comprises compressing the cup edges together when crosslinking to join the cups.
The present invention also discloses a mold for making a golf ball having a multi-layer core comprising a bottom mold plate with a plurality of hemispherical cavities having a cavity diameter and a circumferential groove surrounding each cavity having an inner and outer diameter, the inner diameter being greater than the cavity diameter; a top mold plate with a plurality of hemispherical cavities having a cavity diameter and a circumferential groove surrounding each cavity having an inner and outer diameter, the inner diameter being greater than the cavity diameter; and a center mold plate with a plurality of hemispherical protrusions and a corresponding channel disposed concentrically around the protrusions, the channel having an inner diameter being less than the cavity diameter and an outer diameter greater than the groove inner diameter.
In a preferred method, the mold above is used to form a plurality of core hemispherical cups from elastomeric material by placing the elastomeric material in the cavities of the bottom mold plate and the top mold plate, locating the center mold plate between the top mold plate and the bottom mold plate; and compressing the mold plates to form the cups in the bottom and top mold plate cavities, the cups having substantially hemispherical cavities and a lip extending into the groove such that the lips are coupled to the cups to maintain the cups shape. The center mold plate is removed from between the top and bottom mold plates. The solid or fluid centers are placed in the cups in the top mold plate and the bottom mold plate is located over the top mold plate. Two cups are joined around the centers to form substantially spherical cores and a cover is molded around the cores.
In a one embodiment, the channel has a center diameter that is substantially the same as the cavity diameter. The channel has a depth that is preferably greater than about 0.01 inches. More preferably, the depth is from about 0.01 to 0.05 inches. Most preferably, the depth is from about 0.02 to about 0.03 inches. In another embodiment, the channel inner diameter is between about 95% and 99.5% of the cavity diameter. The channel outer diameter is also preferably between about 100.5% and 105% of the groove inner diameter.
The channel has a first and second sidewall that converge toward the center of the channel, each having angles that are less than 90 degrees from horizontal. More preferably, the angles are from about 30 to 60 degrees from horizontal. Most preferably, the angles are from about 40 to 50 degrees from horizontal. The channel, therefore, can be defined, in one embodiment, by a truncated cone cross-section. In another embodiment, the first and second sidewalls are each defined by an arc having a radius.
Additionally, the channel has a top width and a bottom width smaller than the top width. The bottom width is preferably from about 0.1 to 0.15 inches. More preferably, the bottom width is from about 0.12 to 0.13 inches.
In a preferred method according to the invention, the bottom mold plate is coupled to the center mold plate and the top mold plate. To mold the cups, the center mold plate is folded over the top mold plate. Subsequently, the top mold plate and the center mold plate are folded over the bottom mold plate. The plates are then compressed to form the cups.
In one embodiment, the center mold plate further includes providing hemispherical protrusions that are substantially the same size. Alternatively, the hemispherical protrusions are replaceably connected to the center mold plate.
In one embodiment, the step of compressing the mold plates further includes elevating the temperature of the elastomeric material to a first temperature that is less than the cure activation temperature of the material so that the material becomes pliable. Then, the step of joining the cups includes elevating the temperature of the material to a second temperature greater than the cure activation temperature to crosslink the material.
The present invention is further directed to an apparatus for molding a sphere having a center and a shell, the apparatus comprising a bottom mold plate with a plurality of hemispherical cavities having a cavity diameter and a circumferential groove surrounding each cavity having an inner and outer diameter, the inner diameter being greater than the cavity diameter, a top mold plate with a plurality of hemispherical cavities having the same cavity diameter and a circumferential groove surrounding each cavity having the same inner and outer diameter, and a center mold plate with a plurality of hemispherical protrusions and a corresponding channel disposed concentrically around the protrusions, the channel having an inner diameter being less than the cavity diameter and an outer diameter greater than the groove inner diameter.
In one embodiment, the channel has a center diameter that is substantially the same as the cavity diameter. The channel has a depth that is preferably greater than about 0.01 inches. More preferably, the depth is from about 0.01 to 0.05 inches. Most preferably, the depth is from about 0.02 to 0.03 inches. In another embodiment, the channel inner diameter is between about 55% and 99.5% of the cavity diameter. The channel outer diameter is preferably between about 100.5% and 105% of the groove inner diameter.
In another embodiment, the channel has a first and second sidewall, each having angles that are less than 90 degrees from horizontal, as measured from the center of the channel. More preferably, the angle is from about 30 to 60 degrees from horizontal. Most preferably, the angle is from about 40 to 50 degrees from horizontal. The channel, therefore, can be defined, in one embodiment, by a truncated cone cross-section. In another embodiment, the first and second sidewalls are each defined by an arc having a radius.
In a further embodiment, the channel has a top width and a bottom width smaller than the top width. The bottom width is preferably from about 0.1 to 0.15 inches. More preferably, the bottom width is from about 0.12 to 0.13 inches.